1. Field of the Invention
The present invention relates to an apparatus for controlling an air-fuel ratio in an internal combustion engine using an air-fuel ratio sensor such as a titania (TiO.sub.2) type O.sub.2 sensor associated with an electric heater, and more particularly, to controlling the power supplied to the heater.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing an oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.x simultaneously from the exhaust gas.
As the above-mentioned O.sub.2 sensor, a titania (TiO.sub.2) type O.sub.2 sensor having a high response characteristic is used. Namely, the element resistance of the titania O.sub.2 sensor is small when the air-fuel ratio is rich, and is large when the air-fuel ratio is lean. The element resistance of the titania type O.sub.2 sensor, however, is affected strongly by the temperature thereof, compared with zirconia type O.sub.2 sensors; i.e., when the temperature of the titania type O.sub.2 sensor is increased, an output thereof indicating a lean state is close to that indicating a rich state, and as a result when the above-mentioned air-fuel ratio feedback control is carried out, the controlled air-fuel ratio may be overlean, thus increasing NO.sub.x emissions, and inviting knocking, misfiring, and the like. Therefore, it is important to maintain the titania type O.sub.2 sensor at a high predetermined temperature. Note, such a high temperature state can be detected by incorporating a temperature sensor but this increases the manufacturing cost.
In a prior art, an electric heater is incorporated into an O.sub.2 sensor, and the resistance value of the electric heater is controlled to a definite value (see JP-A-57-197459). Namely, since the temperature of the heater has a definite relationship to the resistance value thereof, and the element temperature of the O.sub.2 sensor also has a definite relationship to the temperature of the heater, the element temperature of the O.sub.2 sensor can be made definite by making the resistance value of the heater definite. Therefore, in this prior art, a supply power supplied to the heater is controlled so that the resistance value of the heater is brought close to a definite value, to thereby keep the element temperature of the O.sub.2 sensor at a definite value.
On the other hand, when the driving state of the engine is determined, a supply of power to the heater required to maintain the temperature of the heater at a definite value is also determined. Thus, in another prior art (see JP-A-60-214251), an aimed supply power is first experimentally obtained for predetermined driving parameters of the engine, and the actual supply of power to the heater is controlled so that the actual power supplied is brought close to the aimed power supplied for the predetermined driving parameters of the engine.
In the above-mentioned prior art, the element temperature of the O.sub.2 sensor can be maintained at a definite value while the engine is in a steady state, but when a transient state such as an acceleration state or a deceleration state of the engine occurs, it is impossible to maintain the element temperature of the O.sub.2 sensor at the definite value for some time after the transient state, and thus a deviation of the controlled air-fuel ratio from the predetermined air-fuel ratio such as the stoichiometric air-fuel ratio occurs, to thereby increase the HC, CO, and NO.sub.x emissions. Also, it is impossible to maintain the element temperature of the O.sub.2 sensor at the definite value after the elapse of a long time, thus also causing a deviation of the controlled air-fuel ratio from the predetermined air-fuel ratio. Further, even when the element temperature of the O.sub.2 sensor can be maintained at the definite value, the resistance value thereof per se may be changed, and thus a deviation of the controlled air-fuel ratio from the predetermined air-fuel ratio occurs.